Bifunctional Chiral Quaternary Ammonium Salt Catalysts: A Rapidly Emerging Class of Powerful Asymmetric Catalysts

Chiral quaternary ammonium salt catalysts have emerged as one of the outstanding catalytic principles over the last decades. Their ability to activate nucleophiles in a noncovalent fashion makes them unique. However, over the last years it has become obvious that in a variety of difficult transformations activation and control of nucleophiles alone does not result in sufficient selectivities, and the same holds true for the sole control of electrophiles by other catalytic motifs. Thus, the use of bifunctional ammonium salt catalysts has become one of the most promising strategies to carry out demanding reactions in a highly stereoselective fashion under mild reaction conditions, and many of these transformations are not possible by using other methods. This review will highlight some of the most impressive recent examples in this field and compare the performances of these catalysts with those of other commonly employed ones.